Trim and damping cylinder

ABSTRACT

A centering and damping cylinder which includes a primary piston for damping and a secondary piston for trim, each piston being of the differential area type and slidable within an internally stepped cylinder housing. In addition to a damping orifice in the primary piston, an internal relief valve may be provided within the cylinder for relieving pressure when the damping orifice cannot handle the required damping flow rate.

0 United States Patent [151 3,677,488 Boehringer [4 1 July 18, 1972 [54]TRIM AND DAMPING CYLINDER 3,458,016 7/1969 Keech ..l88/317 x [72]Inventor: Wilfred E. Boehringer, Fullerton, Calif. Primary ExaminerMilton Buchler 73] Assignee: McDonnell Douglas Corporation AssistantExaminer-Paul E. Sauberer [22] Filed. Feb 16 1971 Attorney-Walter J.Jason, Donald L. Royer and George W.

Finch [21] Appl. No.: 115,229

ABSTRACT 133/230 A centering and damping cylinder which includes aprimary [51] lnLCI. ..B64c 25/34 piston f damping and a secondary pistonf trim, each [58] Fleld of Search ..92/62; 91/ 167; 188/280, 318, pistonbeing ofthe differential area type and slidable within an 188/317 282;244/103 102 104 internally stepped cylinder housing. In addition to adamping 50 orifice in the primary piston, an internal relief valve maybe provided within the cylinder for relieving pressure when the [56]References cued damping orifice cannot handle the required damping flowUNITED STATES PATENTS rate- 3,288,036 11/1966 Fisher ..92/62 15 Claims,5 Drawing Figures Patented July 18, 1972 3,677,488

2 Sheets-Sheet 1 m4 75 54 w M 7% w mm 65 I NVENTOR. W/mzm flay/W652Patented July 18, 1972 2 Sheets-Sheet 2 mm i QQW w Q wm.

w wk III/I TRIM AND DAMPING CYLINDER BACKGROUND OF THE INVENTION bindtherein can cause the uncovering of one or more seals and the failure ofthe device.

The known externally pressurized self-centering dampers,

on the other hand, have the disadvantage of requiring relatively largeports and hydraulic lines since the damping action thereof is generatedby externally exhausting working medium from one pressurizing portthereof into another. This is particularly disadvantageous when thedampers are used to trim the landing gear bogies of large aircraft wherethe fluid flow rate required for the needed damping becomes greater thanthe flow rate that is possible through reasonably sized pressurizingports and interconnecting hydraulic lines.

BRIEF SUMMARY OF THE INVENTION The present centering and dampingcylinder is designed primarily for use in damping vibrations andtrimming aircraft landing gear bogies but it is adaptable to many otherapplications where a source of pressurized medium is available. Thecylinder, as usually installed, levels or positions the bogie forlanding gear retraction and absorbs energy tending to pitch the bogieduring the landing touchdown. The cylinder also includes means whichenable the drooping of the bogie for a specific gear retraction envelopeor to absorb additional impact energy during landing touchdown.

The present cylinder is of the externally pressurized type so that theinherent problems of scaled internally pressurized dampers, discussedabove, are eliminated. The cylinder is designed so that all high ratefluid flow can be ported internally in the cylinder through an internalrelief valve and therefore even excessive damping flows do not have tobe exhausted out of the cylinder ports as is typical of the prior artexternally pressurized self-centering dampers.

The cylinder includes a primary piston for damping and a secondarypiston for trim or centering with each piston being of the differentialarea type and slidable within an internally stepped cylinder housing.When the cylinder is installed as a bogie damper, the primary piston andits connecting rod are usually attached to the landing gear strut whilethe cylinder housing is attached to the bogie. A pair of pressure portscommunicate hydraulic pressure from the system that powers the landinggear extend and retract system to both sides of the primary piston whilean optional drooping pressure port communicates controlled hydraulicpressure to the interior of the secondary piston. Differential areasbetween the stepped cylinder and the primary and secondary pistons areso arranged that the pressure admitted through the pair of ports tendsto move the primary piston in a retract direction and the secondarypiston in the opposite or extend direction. The operative working areaof the secondary piston exceeds that of the primary piston and thereforewhen the two pistons come into contact, the secondary piston moves orbrings the primary piston to a stop at a point determined by the steppedportion of the cylinder housing which prevents further movement of thesecondary piston in the extend direction. When the bogie is to bedrooped, pressure is admitted to the optional drooping port which causesthe secondary piston to move away from the internal step in the retractdirection thereby freeing the primary piston to retract even further. Ifno bogie droop is desired, the drooping port is vented.

have been designed in a nonfailsafe manner so that a minor During alanding the primary piston normally provides damping action by means ofat least one damping orifice therethrough or thereabout, through whichthe hydraulic fluid is forced by movements of the bogie with respect tothe landing gear strut. If during landing the pressures within thecylinder reach too high a level, an internal high flow rate relief valveopens to relieve the pressure across the primary piston. Therefore allhigh flow rates are bypassed from one side of the primary piston to theother within the cylinder and only the volume of fluid for the centeringdifferential area must be exhausted and resupplied to the cylinderthrough the pair of pressure ports. Since centering can be a relativelyslow process and since the centering differential area need not begreat, the ports can have a relatively low flow rate capacity. The portsmay includeoriflces to prevent high pressures from feeding back into thehydraulic system.

The present cylinder is designed to be failsafe so that no matter whathangs up therein,- no seals are uncovered to allow massive leakage ofhydraulic fluid. The pressurized fluid for the cylinder, used incentering to trim or droop the bogie for retraction into the fuselage,is usually supplied by the landing gear retraction hydraulic system. Thecylinder is designed so that pressure in the system sufficient toretract the landing gear is also sufficient to enable the cylinder toproperly position the bogie for retraction. It is therefore highlyunlikely that an unu'immed bogie will even be retracted. This isdesirable since the retraction of untrimmed bogies is a primary cause ofstructural damage to the landing gear wells of aircraft. Also, since thepresent cylinder is backed up by the relatively massive hydraulic systemfor landing gear retraction, major leaks within the trim cylinder can betolerated since the possible volume of leakage is negligible withrespect to the volume of the entire system. Therefore, an aircraft canbe flown with damaged or destroyed seals in its bogie trim cylinderswithout fear of damage to the aircraft.

It is therefore an object of the present invention to provide a reliableself-centering damper.

Another object is to provide means for damping and absorbing energy froma landing gear bogie upon touchdown of the aircraft.

Another object is to provide a trim cylinder which can safely operatewith damaged or worn seals therein.

Another object is to provide a failsafe bogie trim cylinder.

Another object is to provide a bogie trim cylinder which assures thatthe landing gear bogie is in proper position for retraction and/orlanding.

Another object is to provide a self-centering two-way damping cylinderarranged so that high flow rates need not be exhausted during thedamping cycle.

Another object is to provide a bogie trim cylinder which can be used todroop as well as level a landing gear bogie.

These and other objects and advantages of the present invention willbecome apparent after considering the following detailed specificationwhich covers a preferred embodiment thereof in conjunction with theaccompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a partially cutaway view ofa landing gear including a bogie with the present trim and dampingcylinder installed thereon;

FIG. 2 is an enlarged partial cross-sectional view of the presentinvention in a centered condition;

FIG. 3 is a cross-sectional view similar to FIG. 2 but with the cylinderin a retracted condition;

FIG. 4 is also a cross-sectional view similar to FIG. 2 with thecylinder in an extended condition; and

FIG. 5 is a further enlarged cross-sectional view of relief valve meanswithin the cylinder assembly.

DESCRIPTION OF THE SHOWN EMBODIMENT Referring to the drawings moreparticularly by reference numbers, number in FIG. 1 refers to a trim anddamping cylinder constructed according to the present invention. Thecylinder 10 is shown connected to the landing gear strut 11 of anaircraft landing gear assembly 12 by means of bearing means 13 at theend of the cylinders piston rod 14 whereas the cylinders housing 16 isconnected to the bogie 18 of the landing gear assembly 12 by means ofbearing means 19.

The bogie 18 is connected for rotation to the landing gear strut 11 bymeans of a pivot 20. The bogie 18 is pivotally mounted to landing gearstrut 11 so that the front and back wheels, 22 and 24 respectively,connected thereto can conform to imperfections in the runway surfacewithout applying substantial bending loads to the strut 11. As will beshown, the cylinder 10 is used to dampen vibrations between the bogie 18and the strut 11 and to position the bogie 18 at one or morepredetermined angles thereto.

The cylinder 10 is usually used to level the bogie for landing gearretraction and touchdown and to absorb energy tending to pitch the bogieduring the landing touchdown. In addition, the cylinder 10 can be usedto droop either the front or the back wheels, 22 and 24 respectively,for a specific gear retraction envelopeor to absorb additional impactenergy during landing touchdown. In FIG. 1 the cylinder 10 is showninstalled above the bogie 18 between the front wheels 22, however, itshould become immediately apparent that the cylinder 10 can be installedin many other positions between the bogie l8 and the landing gear strutl 1.

Referring to FIGS. 2 through 4, it can be seen that the cylinder housing16 includes an internal sleeve gland 28 which forms one barrier for highpressure fluid therewithin and another gland 30 threadably attached tothe housing 16 at the opposite end thereof. Adjacent to the gland 28 isa first internal cylindrical surface 32 of the housing 16 while thegland 30 is adjacent a second internal cylindrical surface 34 of thehousing 16 having a smaller diameter than that of the cylindricalsurface 32.

The piston rod 14 includes a tail rod portion 36 and a fore rod portion38. The larger diameter tail rod portion 36 rides within the sleevegland 28. A packing 40 is provided to seal the space between the gland28 and the tail rod portion 36 as the piston rod 14 slides with respectto the cylinder housing 16. The smaller diameter fore rod portion 38 ina like manner passes through gland 30 with a packing 42 therebetween toform a seal.

The primary piston 44 of the cylinder 10 is positioned between the tailand fore rod portions, 36 and 38, of the piston rod 14. The piston 44slides on the inner cylindrical surface 34 and includes a packing orring 46 therebetween to prevent substantial amounts of leakage betweenthe piston 44 and the surface 34.

Also within the housing 16 is a secondary sleeve piston 48 which isconstructed to slide on both the inner cylindrical surfaces 32 and 34 ofthe housing 16. The sleeve piston 48 includes packings 50 and 52 to formhydraulic seals between the sleeve piston 48 and the surfaces 32 and 34respectively. The sleeve piston 48 also includes abutment surfaces '54and 56 at the opposite ends thereof. The abutment surface 56 is adaptedto engage a similar abutment surface 58 on the primary piston 44 so thatwhen the primary piston 44 is moved to the left past a predeterminedcenter position, as shown in FIG. 3, the sleeve piston 48 is moved tothe left also. The sleeve piston 48 can be moved to the left until theabutment surface 54 thereof engages an abutment surface 57 on the sleevegland 28.

The sleeve piston 48 also includes another abutment surface 60 which isadapted to engage a similar abutment surface 62 which forms the stepbetween the cylindrical surfaces 32 and 34. When the primary piston 44is moved to the right past the predetermined center position, theabutment surfaces 60 and 62 engage prohibiting the sleeve piston 48 frommoving further to the right. This is shown in FIG. 4.

When in operation damping, the present cylinder 10 is pressurized withhydraulic fluid by means of ports 64 and 65 which are both connected tothe same source of hydraulic pressure 66 by line 67 so that the samepressure is applied to both sides of the primary piston 44. One or moreorifices 68 form passageways through the piston 44 so that when it ismoved in either direction by forces exerted between the housing 16 andthe piston rod 14, hydraulic fluid flows therethrough to damp out themotion.

The centering action of the present cylinder 10 can be seen more clearlyby referring to FIGS. 3 and 4. In FIG. 3 the piston rod 14 is partiallyretracted and must move to the right to be centered. The cross-sectionalarea 69 of the primary piston 44 between the cylindrical surface 34 andthe fore rod portion 38 is smaller than the cross-sectional area 70defined between the cylindrical surface 32 and the tail rod portion 36.This difference in area results in more net force on the left side ofthe primary piston 44 which is in abutment with the sleeve piston 48 sothat the piston rod 14 is forced to the right. When the rod 14 reachesthe position shown in FIG. 2, the sleeve piston 48 can no longer move tothe right since its abutment surface 60 comes in contact with theabutment surface 62 of the housing 16. The working area of the primarypiston 44 is therefore reduced to the cross-sectional area 72 definedbetween the tail rod portion 36 and the cylindrical surface 34. Theworking area 72 is less than the area 69 tending to move the piston 44to the left so the piston 44 and the connected piston rod 14 remain inthe center position.

When the piston rod 14 is in the extended position shown in FIG. 4, thedifference between working areas 69 and 72 causes the primary piston 44and its connected piston rod 14 to move to the left until centered atwhich time the abutment surface 58 of the primary piston 44 comes incontact with the abutment surface 56 of the sleeve piston 48. The piston44 moves no further to the left since to do so would require thatabutment surfaces 60 and 62 separate which would increase the workingarea of the piston 44 on the left side to area 70 which is greater thanthe area 69. It should therefore be obvious that when normallyconfigured, the piston rod 14 will always tend to resume its centeredposition.

As aforesaid, there are instances when it is desired to have the pistonrod 14 assume a stable position other than the centered position. Atthese times, pressure is fed from the source 66 through line 73 and anotherwise normally vented port 74, into a chamber 76 formed between thesleeve piston 48, the cylindrical surfaces 32 and 34. When pressure isadmitted through port 74 it equalizes the forces acting on the sleevepiston 48 so that the primary piston 44 can move it to the left. Thesleeve piston 48, primary piston 44 and piston rod 14 continue to moveto the left until the abutment surface 54 of the sleeve piston 48 comesin contact with abutment surface 57 which is formed integrally with thesleeve gland 28. Therefore, by admitting pressure by means of port 74,the piston rod 14 can be made to assume a stable retracted position. Ifthe cylinder 10 is properly connected to the bogie 18, this causes thebogie to droop either for a specific gear retraction envelope or so thatmore impact energy can be absorbed by the landing gear assembly 12 thanis otherwise possible with a more level bogie at touchdown.

A high flow rate relief valve assembly 54, shown in detail in FIG. 5, isprovided within the cylinder assembly 10 to provide means for relievingthe high internal pressures generated by rapid or large angularmovements of the bogie 18 with respect to the strut 11, such as normallyoccur during the touchdown impact. These high pressures are generatedwhen the position of the bogie is abruptly changed through a large anglebecause the orifices 68 in the primary piston 44 are sized primarily forthe relatively smaller flows involved in pitch damping during normalsink rate landings.

When the primary piston 44 is moved to the left fast enough to generatethe above mentioned high pressures, the hydraulic fluid from the leftside of piston 44 is conducted through a plurality of openings 86 in thetail rod portion 36. From the openings 86 the fluid is conducted througha passageway 88 formed between the inner surface of the tail rod portion36 and the housing 90 of the relief valve assembly 84. From thepassageway 88 the fluid is conducted through openings 92 in the housing90 and openings 94 in a valve end gland member 96 positioned as shown.The high pressure caused by the leftward movement of the primary piston44 then acts against the end 98 of a plunger 100 centrally locatedwithin the gland member 98. The plunger 100 is constructed so that itcan be moved to the right when the force exerted by the hydraulic fluidis sufficient to compress a relatively heavy spring 102 which bearsthereagainst.

When the plunger 100 moves to the right, the end 98 thereof lifts off anassociated seat member 104 thereby allowing flow of the hydraulic fluidthrough a passageway 106 in the plunger 100. The fluid then flowsthrough another passageway 108 in a second plunger 110, similar toplunger 100, which acts against the face of a seat member 112 similar tothe seat member 104. The seat members 104 and 112 are both lightlyspring loaded toward their associated plungers by springs 114 and 116respectively and they are both adapted for movement outward from theplungers 100 and 110 when a pressure differential acting in an outwarddirection appears thereacross. It should be noted that, due to theirshapes and back side areas, a pressure differential acting inward nevercauses them to lift off their associated plungers. Therefore, thehydraulic fluid passing through the passageway [08, causes the seatmember 112 to unseat from the end 118 of the plunger 110. From there,the fluid passes through a spider 120 which supports the seat member 112and a plurality of openings 122 in the fore rod portion 38 of the pistonrod 14 to thereby relieve the pres sure on the left side of the primarypiston 44.

When the high pressure is on the right side of the primary piston 44,the hydraulic fluid flows through the openings 122, the spider 120 andbears against the end 118 of the plunger 110 to move the plunger 110against spring 102 and away from the seat member 112. The fluid thenflows through the passageways 108 and 106, past the seat member 104which moves back against its spring 114, through the openings 94, 92,the passageway 88 and the opening 86 to the opposite side of the primarypiston 44.

The relief valve assembly 84 therefore internally bypasses all high rateflows so that the supply ports 64 and 65 only need be large enough tosupply centering fluid which, as aforesaid, is not required at high flowrates.

Thus, there has been shown and described a novel trim and dampingcylinder which fulfills all of the objects and advantages soughttherefor. Many changes, modifications, variations, and other uses andapplications of the subject trim and damping cylinder will, however,become apparent to those skilled in the art after considering thespecification together with the accompanying drawings. All such changes,modifications, variations, and other uses and applications which do notdepart from the spirit and scope of the invention are deemed to becovered by the invention which is limited only by the claims whichfollow.

What is claimed is:

l. A trim and damping cylinder comprising:

a container including means to pressurize said container with medium;

piston means within said container dividing said container into firstand second container portions, said piston means when in a predeterminedposition having equal working areas acting on the first and secondcontainer portion sides thereof;

means enabling a restricted flow of pressurized medium between saidfirst and second container portions to dampen motion between saidcontainer and said piston means;

means for increasing the working area of said piston means on the firstcontainer portion side thereof when said piston means are moved towardsaid first container portion from said predetermined position; and

means for decreasing the working area of said piston means on the firstcontainer portion side thereof when said piston means are moved awayfrom said first container portion and said predetermined position;whereby said piston means tend to return to said predetermined position.

2. The cylinder defined in claim 1 including:

means connected to said container to selectively disable said workingarea increasing means and enable said working area decreasing means sothat said piston means tend to move in the direction of said firstcontainer portion of said cylinder.

3. The cylinder defined in claim 1 including:

relief means in communication with said first and second containerportions for relieving predetermined differential pressure between saidcontainer portions.

4. A positioning damper comprising:

a container connected to a source of pressurized medium;

a piston rod extending through a first end of said container;

a piston within said container connected to said piston rod dividingsaid container into first and second container portions;

means enabling a restricted flow of pressurized medium between saidfirst and second container portions to dampen motion between saidcontainer and said piston rod;

means reducing the volume of pressurized medium within said containerwhen said piston is moved from a predetermined position toward saidfirst container end; and

means reducing the volume of pressurized medium within said containerwhen said piston is moved from the predetermined position away from saidfirst container end; whereby said piston tends to remain at thepredetermined position.

5. The damper defined in claim 4 wherein:

said pressurized medium is relatively incompressible.

6. The damper defined in claim 4 wherein said restricted flow enablingmeans include:

at least one orifice through said piston communicating said first andsecond container portions.

7. The damper defined in claim 4 including:

means to selectively neutralize one of said volume reducing means sothat said piston tends to move toward one end of said container.

8. The damper defined in claim 4 including:

means for relieving predetermined differential pressure acting acrosssaid piston, said relief means enabling substantial flow of medium incomparison to the flow of medium enabled by said restricted flowenabling means.

9. The damper defined in claim 8 wherein:

said differential pressure relief means are positioned within saidpiston rod to move therewith.

10. A cylinder comprising:

a cylinder housing having first and second internal cylinder walls, saidfirst cylinder wall having a larger diameter than said second cylinderwall, an abutment surface between said walls defining a center positionreference in said cylinder, and means to feed pressurized medium intosaid cylinder;

a first piston within said cylinder housing riding on said secondcylinder wall;

a connecting rod attached to said first piston, said connecting rodincluding a first rod portion at one side of said first piston and asecond rod portion at the other side of said first piston; and

a second piston within said cylinder housing riding on said first andsecond cylinder walls, said second piston being adapted for abutmentwith said first piston, said second piston also being adapted forabutment with said cylinder housing abutment surface to restrict travelof said second piston in a predetermined direction;

said first cylinder wall and said first rod portion defining a firstworking area, said second cylinder wall and said first rod portiondefining a second working area smaller than said first working area andsaid second cylinder wall and said second rod portion defining a thirdworking area smaller than said first working area but larger than saidsecond working area, said first working area working on said firstpiston in opposition to said third working area when said first pistonis in abutment with said second piston so said fust piston tends to moveaway from said second piston, said second working area working on saidfirst piston in opposition to said third working area when said firstpiston is not in abutment with said second piston, whereby said pistonrod tends to return to a position where said first piston is in abutmentwith said second piston and said second piston is in abutment with saidcylinder housing abutment surface.

11. The cylinder defined in claim 10 wherein:

said second piston and said first and second cylinder walls define aclosed chamber; and said cylinder housing includes:

means to selectively feed pressurized medium in said closed chamber toneutralize a portion of said first working area so that said firstpiston tends to assume a position in abutment with said second pistonwhere said second piston is out of abutment with said cylinder housingabutment surface.

12. The cylinder defined in claim 10 including:

orifice means through said first piston to allow restricted flow ofmedium from one side of said first piston to the other to damp outmotion between said cylinder housing and said piston rod.

13. The cylinder defined in claim 10 including:

relief valve means within said cylinder housing operatively connected torelieve pressure of medium above a predetermined pressure from one sideof said first piston to the other.

14. The cylinder defined in claim 10 including:

means to allow restricted flow of medium from one side of said firstpiston to the other to damp out motion between said cylinder housing andsaid piston rod.

15. The cylinder defined in claim 14 including:

relief valve means within said piston rod to relieve pressure of mediumabove a predetermined pressure from one side of said first piston to theother.

a i t t a:

1. A trim and damping cylinder comprising: a container including meansto pressurize said container with medium; piston means within saidcontainer dividing said container into first and second containerportions, said piston means when in a predetermined position havingequal working areas acting on the first and second container portionsides thereof; means enabling a restricted flow of pressurized mediumbetween said first and second container portions to dampen motionbetween said container and said piston means; means for increasing theworking area of said piston means on the first container portion sidethereof when said piston means are moved toward said first containerportion from said predetermined position; and means for decreasing theworking area of said piston means on the first container portion sidethereof when said piston means are moved away from said first containerportion and said predetermined position; whereby said piston means tendto return to said predetermined position.
 2. The cylinder defined inclaim 1 including: means connected to said container to selectivelydisable said working area increasing means and enable said working areadecreasing means so that said piston means tend to move in the directionof said first container portion of said cylinder.
 3. The cylinderdefined in claim 1 including: relief means in communication with saidfirst and second container portions for relieving predetermineddifferential pressure between said container portions.
 4. A positioningdamper comprising: a container connected to a source of pressurizedmedium; a piston rod extending through a first end of said container; apiston within said container connected to said piston rod dividing saidcontainer into first and second container portions; means enabling arestricted flow of pressurized medium between said first and secondcontainer portions to dampen motion between said container and saidpiston rod; means reducing the volume of pressurized medium within saidcontainer when said piston is moved from a predetermined position towardsaid first container end; and means reducing the volume of pressurizedmedium within said container when said piston is moved from thepredetermined position away from said first container end; whereby saidpiston tends to remain at the predetermined position.
 5. The damperdefined in claim 4 wherein: said pressurized medium is relativelyincompressible.
 6. The damper defined in claim 4 wherein said restrictedflow enabling means include: at least one orifice through said pistoncommunicating said first and second container portions.
 7. The damperdefined in claim 4 including: means to selectively neutralize one ofsaid volume reducing means so that said piston tends to move toward oneend of said container.
 8. The damper defined in claim 4 including: meansfor relieving predetermined differential pressure acting across saidpiston, said relief means enabling substantial flow of medium incomparison to the flow of medium enabled by said restricted flowenabling means.
 9. The damper defined in claim 8 wherein: saiddifferential pressure relief means are positioned within said piston rodto move therewith.
 10. A cylinder comprising: a cylinder housing havingfirst and second internal cylinder walls, said first cylindEr wallhaving a larger diameter than said second cylinder wall, an abutmentsurface between said walls defining a center position reference in saidcylinder, and means to feed pressurized medium into said cylinder; afirst piston within said cylinder housing riding on said second cylinderwall; a connecting rod attached to said first piston, said connectingrod including a first rod portion at one side of said first piston and asecond rod portion at the other side of said first piston; and a secondpiston within said cylinder housing riding on said first and secondcylinder walls, said second piston being adapted for abutment with saidfirst piston, said second piston also being adapted for abutment withsaid cylinder housing abutment surface to restrict travel of said secondpiston in a predetermined direction; said first cylinder wall and saidfirst rod portion defining a first working area, said second cylinderwall and said first rod portion defining a second working area smallerthan said first working area and said second cylinder wall and saidsecond rod portion defining a third working area smaller than said firstworking area but larger than said second working area, said firstworking area working on said first piston in opposition to said thirdworking area when said first piston is in abutment with said secondpiston so said first piston tends to move away from said second piston,said second working area working on said first piston in opposition tosaid third working area when said first piston is not in abutment withsaid second piston, whereby said piston rod tends to return to aposition where said first piston is in abutment with said second pistonand said second piston is in abutment with said cylinder housingabutment surface.
 11. The cylinder defined in claim 10 wherein: saidsecond piston and said first and second cylinder walls define a closedchamber; and said cylinder housing includes: means to selectively feedpressurized medium in said closed chamber to neutralize a portion ofsaid first working area so that said first piston tends to assume aposition in abutment with said second piston where said second piston isout of abutment with said cylinder housing abutment surface.
 12. Thecylinder defined in claim 10 including: orifice means through said firstpiston to allow restricted flow of medium from one side of said firstpiston to the other to damp out motion between said cylinder housing andsaid piston rod.
 13. The cylinder defined in claim 10 including: reliefvalve means within said cylinder housing operatively connected torelieve pressure of medium above a predetermined pressure from one sideof said first piston to the other.
 14. The cylinder defined in claim 10including: means to allow restricted flow of medium from one side ofsaid first piston to the other to damp out motion between said cylinderhousing and said piston rod.
 15. The cylinder defined in claim 14including: relief valve means within said piston rod to relieve pressureof medium above a predetermined pressure from one side of said firstpiston to the other.